The present invention relates to programmable logic devices, more particularly to redundant circuitry for replacing defective circuits.
Programmable logic devices contain circuits that may be configured by a user to provide various logic functions. During the process of manufacturing such devices, an occasional defect may develop which renders the circuit inoperable. Sometimes the defect is serious enough that the device must be discarded. However, if only a small portion of the circuitry is affected, the device can often be repaired by switching redundant circuitry into use in place of the defective circuitry.
An example of a programmable logic device that has redundant circuitry is shown in So et al. U.S. Pat. No. 4,899,067. The device shown in the So et al. patent has sum-of-products logic which is subdivided into relatively small units called macrocells. When a macrocell is found to be defective it is replaced by switching in a nearby redundant macrocell. This approach allows devices with macrocell defects at various locations across the device to be repaired. However, defects often occur only at relatively localized areas on the device. In that case, much of the circuitry that is used to switch the redundant macrocells into place is not required. If the unneeded switching circuitry could be eliminated, the programmable logic device would have improved performance and would be less expensive to manufacture.
It is therefore an object of the present invention to improve and simplify the provision and use of redundant circuits in programmable logic devices.
It is another object of this invention to provide programmable logic devices with redundant circuits for replacement of defective circuits on a relatively large scale so that the overhead associated with controlling the use of the redundant circuits can be substantially reduced.